Glioblastoma (GB) is the most common form of primary adult brain tumors characterized by a poorly defined tumor mass resulting from highly invasive cells. The problem of resistance to the standard anti-proliferative treatment of concomitant radiotherapy with chemotherapy using the alkylating agent temozolomide (TMZ) is common, and actively invading cells survive the current therapeutic regimens. Glioma cells with the increased capacity for migration have a decreased expression of pro-apoptotic genes and are less sensitive to cytotoxic therapy-induced apoptosis (1-4); the knockdown of several pro-invasive gene candidates in GB decreases glioma cell migration rate and subsequently sensitizes the cells to cytotoxic therapy and importantly, therapy directed at mediators of invasion has been shown to increase chemotherapeutic sensitivity (5-7).
An increased capacity for cell survival results from the multi-faceted regulation of pathways involved in promoting cell growth, replication and spread, and preventing apoptosis in response to cytotoxic insult (8). Treatment strategies of tumor irradiation and temozolomide administration in glioblastoma lead to the formation of DNA double strand breaks (DSBs), either directly, or via mismatch repair conversion of O(6)-methylguanine adducts into DSBs, respectively (9). DSBs are primarily repaired through two mechanisms, homologous recombination (HR) and non-homologous end-joining (NHEJ). HR repair makes use of a non-damaged homologous DNA template, and thus is characterized as an error free mechanism, while NHEJ has no homologous strand for template use resulting in sequence errors near the break point (10).
DNA repair is initiated via sensing of DSBs by three kinases: ataxia telangiectasia mutated (ATM), ataxia telangiectasia and Rad3 related (ATR), and Chk2. Subsequently, the early phosphorylation of histone H2A.X (γH2A.X) by ATM occurs at damaged DNA foci and leads to the phosphorylation of mediator of DNA damage checkpoint protein 1 (MDC1), with subsequent chromatin remodeling and recruitment of DNA repair proteins (11). BRCA1 is one such key mediator of HR and NHEJ repair; after exposure to DNA damaging agents BRCA1 is rapidly phosphorylated by ATM, ATR, and Chk2, and relocated to sites of replication forks with γH2A.X foci, where it recruits further proteins including BRCA2 and Rad51 to mediate strand exchange toward DNA repair and cell survival (10).
One key driver in GB that has been characterized to promote both cell invasion and cell survival is the transmembrane receptor fibroblast growth factor inducible-14 (Fn14). Fn14 is a member of the tumor necrosis factor receptor superfamily with one known ligand, the tumor necrosis factor-like weak inducer of apoptosis (TWEAK). Signaling through Fn14 by its cytokine ligand TWEAK activates Rac1, Akt, and NF-κB-pathways, and has been shown to promote increased cell invasion and resistance to cytotoxic therapy-induced apoptosis (3, 4, 12).
As such, there is a demonstrated need to further investigate in GB systems molecules that not only promote GB's survival during chemotherapy treatment, but is also itself an attractive target for therapeutic targeting.